Intra-cavity frequency converted (frequency doubled) OPS-lasers can provide several Watts (W) of continuous wave (CW) radiation output at visible (green) wavelengths while operating in a single longitudinal mode. The green, single-mode output wavelength can be converted to a wavelength in the ultraviolet (UV) region of the electromagnetic spectrum by further frequency multiplication in an optically non-linear crystal outside the OPS laser cavity (laser-resonator).
By way of example, an OPS-laser having a fundamental lasing wavelength of about 1064 nanometers (nm) can be frequency-doubled by an intra-cavity optically nonlinear crystal to provide output radiation having a wavelength of about 532 nm. That output radiation can be converted to UV radiation having a wavelength of about 266 nm by frequency-doubling the output radiation in an optically nonlinear crystal located outside the OPS laser-resonator.
OPS lasers employ a multilayer semiconductor structure as a gain-element (gain structure). The gain-structure includes quantum-well (QW) layers spaced apart by spacer layers. The spacer layers absorb optical pump radiation thereby producing electron-hole pairs. The electron-hole pairs fall into, and are confined by, the QW layers. Recombination of electron-hole pairs in the QW layers provides radiation in a fundamental wavelength range characteristic of the QW material of the QW layers. This provides optical gain. The wavelength range in which radiation is produced is referred to as the gain-bandwidth.
The QW layers of the gain-structure are spaced apart by one-half wavelength at the peak gain-wavelength of the gain-bandwidth. In an OPS-structure (OPS-chip), the gain-structure surmounts a mirror-structure. Typically an OPS-laser resonator is configured with the mirror of the OPS-chip providing one end-mirror of the resonator. If the resonator is configured to avoid supporting lateral oscillation modes (transverse modes), placement of the gain-structure at an end of the resonator together with the half-wave periodicity of gain provided by the spaced-apart QW layers provides that the OPS-laser will generate CW radiation in a single longitudinal mode (single-frequency), at any given instant. A discussion of the significance of the statement “at any given instant” is set forth below.
The gain-bandwidth of an OPS gain-structure is relatively very broad. By way of example, for a peak-gain wavelength of about 1000 nm, the FWHM gain bandwidth is on the order of 40 nm. In an intra-cavity frequency doubled OPS-laser it is usual to include a single element birefringent filter in the cavity. This restricts the range of fundamental wavelengths that can oscillate to a range within a phase-matching acceptance bandwidth for the optically nonlinear crystal. The birefringent filter is arranged for Brewster angle incidence, which establishes the polarization-orientation of the circulating radiation generated in the resonator.
Unfortunately, the pass-bandwidth of such a birefringent filter is still sufficiently broad that in a resonator having a length of about 70 millimeters (mm) there may be several possible oscillating modes (frequencies). A result of this is that, while only one of these modes oscillates at any given instant, any perturbation of the resonator can result in oscillation changing from one possible mode to an adjacent possible mode. This phenomenon is usually referred to by practitioners of the art as “mode hopping”. At the instant of the mode hop there is an abrupt (essentially instantaneous) change in output power followed by a brief period of instability. This instability period will be on the order of a few hundredths of a second. This constitutes effectively an interruption of the CW output of the OPS-laser. If an actively controlled external enhancement resonator is used for further frequency-conversion of the OPS-laser output, there will be an abrupt drop in output-power from that resonator, and a recovery period during which the enhancement resonator establishes resonance for the new output-frequency of the laser. This will be longer than the instability period for the laser output power.
FIG. 1 is a reproduction of an oscilloscope trace of monitored output-power of an intra-cavity frequency-doubled OPS laser that has been perturbed by increasing output power from 90% to 110% of a nominal optimum output-power. The power increase is initiated at point A. The laser operates at the increased power for about 3.5 seconds then mode-hop occurs at point B, at which output is interrupted. Output-power resumes at a higher level due to stored energy in the gain-structure during the interruption. The period of instability following the interruption is about 1.5 seconds.
In applications such as inspection of semiconductor wafers such an interruption of the CW process may be intolerable, even if the wavelength change due to the mode-hop is tolerable. There is a need for an OPS laser-resonator that can operate without mode-hopping.